210 research outputs found
Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo
We find that the relative contribution of satellite galaxies accreted at high
redshift to the stellar population of the Milky Way's smooth halo increases
with distance, becoming observable relative to the classical smooth halo about
15 kpc from the Galactic center. In particular, we determine
line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence
turnoff (MPMSTO) population along every Sloan Extension for Galactic
Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting
our sample to those lines of sight along which we do not detect elements of
cold halo substructure (ECHOS), we compile the largest spectroscopic sample of
stars in the smooth component of the halo ever observed in situ beyond 10 kpc.
We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population
in the distant half of our sample beyond about 15 kpc from the Galactic center.
Inside of 15 kpc however, we find no significant spatial autocorrelation in
[Fe/H]. At the same time, we perform SEGUE-like observations of N-body
simulations of Milky Way analog formation. While we find that halos formed
entirely by accreted satellite galaxies provide a poor match to our
observations of the halo within 15 kpc of the Galactic center, we do observe
spatial autocorrelation in [Fe/H] in the simulations at larger distances. This
observation is an example of statistical chemical tagging and indicates that
spatial autocorrelation in metallicity is a generic feature of stellar halos
formed from accreted satellite galaxies.Comment: 27 pages, 8 figures, and 7 tables in emulateapj format; accepted for
publication in ApJ. Full tables can be extracted from LaTeX sourc
Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars
We describe a method for the determination of stellar [C/Fe] abundance ratios
using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By
means of a star-by-star comparison with a set of SDSS/SEGUE spectra with
available estimates of [C/Fe] based on published high-resolution analyses, we
demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to
a precision better than 0.35 dex. Using the measured carbon-to-iron abundance
ratios obtained by this technique, we derive the frequency of carbon-enhanced
stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars
and other samples from the literature. We find that the differential frequency
slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a
sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to
[Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars
varies across different luminosity classes. The giant sample exhibits a
cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and
33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower
cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population
displays a large change in the cumulative frequency for CEMP stars below [Fe/H]
= -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When
we impose a restriction with respect to distance from the Galactic mid-plane
(|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low
metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of
C-rich material in stars that have undergone mixing with CNO-processed material
from their interiors. The frequency of CEMP stars near the main-sequence
turnoff, which are not expected to have experienced mixing, increases for
[Fe/H] < -3.0. [abridged]Comment: 19 pages, 10 figures, 6 tables, accepted for publication in AJ on
August 20, 201
Advances in instrumentation at the W. M. Keck Observatory
In this paper we describe both recently completed instrumentation projects and our current development efforts in the context of the Observatory's science driven strategic plan which seeks to address key questions in observational astronomy for extra-galactic, Galactic, and planetary science with both seeing limited capabilities and high angular resolution adaptive optics capabilities. This paper will review recently completed projects as well as new instruments in development including MOSFIRE, a near IR multi-object spectrograph nearing completion, a new seeing limited integral field spectrograph for the visible wavelength range called the Keck Cosmic Web Imager, and the Keck Next Generation Adaptive Optics facility and its first light science instrument DAVINCI
Metallicity Gradients in the Milky Way Disk as Observed by the SEGUE Survey
The observed radial and vertical metallicity distribution of old stars in the
Milky Way disk provides a powerful constraint on the chemical enrichment and
dynamical history of the disk. We present the radial metallicity gradient,
\Delta[Fe/H]/\Delta R, as a function of height above the plane, |Z|, using 7010
main sequence turnoff stars observed by the Sloan Extension for Galactic
Understanding and Exploration (SEGUE) survey. The sample consists of mostly old
thin and thick disk stars, with a minimal contribution from the stellar halo,
in the region 6 < R < 16 kpc, 0.15 < |Z| < 1.5 kpc. The data reveal that the
radial metallicity gradient becomes flat at heights |Z| > 1 kpc. The median
metallicity at large |Z| is consistent with the metallicities seen in outer
disk open clusters, which exhibit a flat radial gradient at [Fe/H] ~ -0.5. We
note that the outer disk clusters are also located at large |Z|; because the
flat gradient extends to small R for our sample, there is some ambiguity in
whether the observed trends for clusters are due to a change in R or |Z|. We
therefore stress the importance of considering both the radial and vertical
directions when measuring spatial abundance trends in the disk. The flattening
of the gradient at high |Z| also has implications on thick disk formation
scenarios, which predict different metallicity patterns in the thick disk. A
flat gradient, such as we observe, is predicted by a turbulent disk at high
redshift, but may also be consistent with radial migration, as long as mixing
is strong. We test our analysis methods using a mock catalog based on the model
of Sch\"onrich & Binney, and we estimate our distance errors to be ~25%. We
also show that we can properly correct for selection biases by assigning
weights to our targets.Comment: Submitted to ApJ; 22 pages, 14 figures in emulateapj format; Full
resolution version available at
http://www.ucolick.org/~jyc/gradient/cheng_apj_fullres.pd
HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions
(HALO7D) survey measures the kinematics and chemical properties of stars in the
Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D
consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured
proper motions of MW halo main sequence turn-off (MSTO) stars in the four
CANDELS fields. HALO7D consists of deep pencil beams, making it complementary
to other contemporary wide-field surveys. We present the [Fe/H] and [alpha/Fe]
abundances for 113 HALO7D stars in the Galactocentric radial range of
kpc. Using the full 7D chemodynamical data (3D positions, 3D
velocities, and abundances) of HALO7D, we measure the velocity anisotropy,
, of the halo velocity ellipsoid for each field and for different
metallicity-binned subsamples. We find that two of the four fields have stars
on very radial orbits while the remaining two have stars on more isotropic
orbits. Separating the stars into high, mid, and low [Fe/H] bins at dex
and dex for each field separately, we find differences in the
anisotropies between the fields and between the bins; some fields appear
dominated by radial orbits in all bins while other fields show variation
between the [Fe/H] bins. These chemodynamical differences are evidence that the
HALO7D fields have different fractional contributions from the progenitors that
built up the MW stellar halo. Our results highlight the additional information
that is available on smaller spatial scales when compared to results from a
spherical average of the stellar halo.Comment: 32 pages, 15 figure
Color Confirmation of Asteroid Families
We discuss optical colors of 10,592 asteroids with known orbits selected from
a sample of 58,000 moving objects observed by the Sloan Digital Sky Survey
(SDSS). This is more than ten times larger sample that includes both orbital
parameters and multi-band photometric measurements than previously available.
We confirm that asteroid dynamical families, defined as clusters in orbital
parameter space, also strongly segregate in color space. In particular, we
demonstrate that the three major asteroid families (Eos, Koronis, and Themis),
together with the Vesta family, represent four main asteroid color types. Their
distinctive optical colors indicate that the variations in chemical composition
within a family are much smaller than the compositional differences between
families, and strongly support earlier suggestions that asteroids belonging to
a particular family have a common origin. We estimate that over 90% of
asteroids belong to families.Comment: 18 pages, color figures, accepted by A
Advances in instrumentation at the W. M. Keck Observatory
In this paper we describe both recently completed instrumentation projects and our current development efforts in the context of the Observatory's science driven strategic plan which seeks to address key questions in observational astronomy for extra-galactic, Galactic, and planetary science with both seeing limited capabilities and high angular resolution adaptive optics capabilities. This paper will review recently completed projects as well as new instruments in development including MOSFIRE, a near IR multi-object spectrograph nearing completion, a new seeing limited integral field spectrograph for the visible wavelength range called the Keck Cosmic Web Imager, and the Keck Next Generation Adaptive Optics facility and its first light science instrument DAVINCI
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